Device for x-ray lithography

ABSTRACT

An invention relates to a contact lithography device and comprises a soft X-ray source (preferable as an X-ray tube  1  with a rotating anode), a half lens  2  for a divergent radiation of this source transforming to a quasi-parallel one (the said lens includes a set of channels for a radiation transporting with total external reflection, the said channels are oriented along a generatrix of barrel-shaped surfaces), the means for placing a mask  3  and a substrate  4  with a resist  5  applied on it (the said means are placed on the output face side of a half lens  2 ), and an absorbing filter  6  for smoothing a nonuniformity of a beam intensity of an emergent radiation of a half lens, presenting in intensity decreasing as from the center to the periphery of a beam. An absorbing filter  6  is placed between the radiation source  1  and the input face of a half lens  2 , and a relationship of cross sizes of a half lens and its focal distance from the input side is chosen so that to provide a capture angle of a source radiation in the following limits: 
     0.7/ E   1.5 ≦ψ≦1.3/ E   1.5 , 
     where  
     ψ is an angle of a radiation capture [rad];  
     E is radiation energy of the used source [keV].  
     A material, reflecting surface of the channels for a radiation transporting is made of, includes elements with an atomic number not more than 22, and radiation energy of the used source is 0.6-6 keV.  
     Owing to the increased capture angle and an absorbing filter placing before the input of a half lens an efficiency of a source radiation using increases and simultaneously an area of a plate-substrate under processing magnifies and lens longevity increases.

[0001] The suggested invention relates to the means for X-ray contactlithography used in microelectronics, namely for lithography devices,which use X-ray lenses to form an X-ray quasi-parallel beam.

[0002] The first information about such devices relates to the end ofeighties. Some works, describing the application of X-ray lenses inlithography devices, one can find in: “Book of Abstracts. 3^(rd)All-Union Conference on Relativistic Particles Radiation in Crystals.May 25 30, 1988”. (Nalchik, 1988) [1]. If a divergent radiation sourceis used these devices comprise a lens, transforming the said radiationto a quasi-parallel one directed through the mask on the resist, appliedon the substrate. The said lens comprises a set of channels for aradiation transporting, curved along barrel-shaped generatrix. An effectof multiple total external reflections from the walls of the channels isused when a radiation is transported. As a whole a lens for a divergentradiation transforming to a quasi-parallel one is half barrel-shaped andit is called a half lens (as against a full lens, which focuses adivergent X-rays and is barrel-shaped).

[0003] More detailed information about the X-ray lithography devicesincluding all elements of such devices mentioned above one can find inthe review “Multiple reflection from surface X-ray optics” (M. A.Kumakhov and F. F. Komarov. PHYSICS REPORTS. A review Section of PhysicsLetters, volume 191, number 5, August 1990. North-Holland) [2], p.345-348).

[0004] Nonuniform radiation intensity (decreasing toward a periphery ofthe output face of a lens), falling on a mask, is a disadvantage of thedescribed devices. It makes necessary to use filters in order to absorb“extra” radiation in the central section of the emergent beam of thelens. This solution was mentioned in source [1], however a structure ofthe device, comprising the said filter, as a whole is described in U.S.Pat. No. 5,175,755 [3] (published 29.12.92). A placement of theabsorbing filter after the half lens (before or after the mask) isdescribed in this patent.

[0005] The second mentioned variant of an absorbing filter placing isobviously poor, as according to this variant a radiation of extraintensity exposures on the central part of the mask, being an expensiveprecise unit, what precipitates the mask destroying. Besides, theinfluence of actual out-of-parallelism of emergent radiation of a halflens appears in this variant to a greater extent. This radiationdivergence determines the more spreading of shadowgraph after the maskthe more is the distance after the mask. Placing the filter after themask inevitably increases this distance.

[0006] The first variant (placing a filter between a half lens and amask) is more desirable. However according to this variant as well aradiation, which extra intensity should be attenuated, passes throughanother expensive unit of the device, an X-ray half lens, whatprecipitates its aging.

[0007] The influence of the aforesaid factors leads to refusal of usageof absorbing filters, in particular if it is necessary “to smooth” greatdifference between a radiation intensity in the central and peripheralparts of the cross-section of the emergent beam, when a radiationintensity, being transported along the central channels of the lens,exceeds a lot a radiation intensity after the filter. This, in its turn,makes necessary to use rather “thin” lenses to make the said differencesmall. Such lenses have a small capture angle of radiation, emergingfrom the source.

[0008] One more factor, fostering the usage of “thin” lenses with arather small capture angle in X-ray lithography, is a sharp drop oftransmission coefficient of a lens at a radiation rotation angle (when ahalf lens is used it is equal to a half of a capture angle) exceedingsome limit value. So, according to calculations (see the results insource [2], p. 318), this value is equal to 0.3-0.4 radian. As thiseffect is well known, it is considered that further increasing of acapture angle will not increase an integral intensity of an emergentbeam. The necessity to suppress a radiation intensity in the center ofthe beam “to equalize” it with a low peripheral level of intensity seemsto be a very strong factor, making no point in using a small captureangle in lithography.

[0009] So, for instance, in the work: M. A. Kumakhov. State andperspectives of capillary Roentgen optics. Proceedings of SPIE—TheInternational Society for Optical Engineering. Volume 2011, Jul. 14-161993, San Diego, Calif. [4] real lithography devices with a captureangle of a half lens from 0.15 up to 0.3 radian are described. Such halflenses capture not more than 1-2% of an isotope source radiation.

[0010] Another result of using “thin” half lenses is a smallcross-section of the emergent beam, what makes possible to irradiateonly small part of the substrate area with a resist, applied on it. Toprocess the whole area it is necessary to use stepwise irradiation. Inspite of using special high-precise devices for this purpose, it isimpossible to avoid errors, caused by errors in conjugation ofneighboring zones under exposure.

[0011] The suggested invention is aimed at obtaining a technical result,implying that the usage of a source radiation increases simultaneouslywith enlarging of the area of a plate under exposure and increasing alens longevity. This technical result can be obtained owing to thecombination of two expedients: an absorbing filter is placed between asource and a half lens and a half lens with larger capture angle isused. For the latter an existence of an optimum value, considerablyexceeding the aforesaid limit values and depending on an energy of theused radiation and material properties of reflecting surfaces of thechannels of a half lens (i.e. a material, the channels are made of ifthey are not applied, or a material applied if exists), has revealed. Inthe energy range of a radiation being used from 0.6 keV up to, at least,6 keV an optimum value of a capture angle depends only on a radiationenergy. A material, a reflecting surface of the channels is made of, canbe any one, feasible from the technological point of view for producingthe channels of an X-ray lens or for their inner surface applying oncondition that it contains only light elements (atomic number should benot more than 22).

[0012] The suggested device for contact lithography comprises as well asthe aforesaid more close to it known device according to U.S. Pat. No.5,175,755 a source of a soft X-rays, a half lens for a divergentradiation of this source transforming to a quasi-parallel one (the saidlens includes a set of channels for a radiation transporting with atotal external reflection, and the said channels are oriented along ageneratrix of barrel-shaped surfaces), the means for placing a mask anda substrate with a resist applied on it being located on the side of anoutput face of a half lens, and an absorbing filter for smoothing anonuniformity of a beam intensity of an emergent radiation of a halflens, manifesting by intensity decreasing from the center to a beamperiphery.

[0013] As against the known one in the suggested device an absorbingfilter is placed between a radiation source and an input face of a halflens, and relationship of the half lens cross sizes and a focal distancefrom the side of an input is chosen for reasons of providing a captureangle of a source radiation in the following limits:

0.7/E ^(1.5)≦ψ≦1.3/E ^(1.5),  (1)

[0014] where

[0015] ψ is a capture angle [rad];

[0016] E is a radiation energy of a used source [keV],

[0017] thus a material of a reflecting surface of the channels for aradiation transporting includes the elements with an atomic number notmore than 22, and a radiation energy of a used source is from 0.6 up to6 keV.

[0018] A relationship (1) is empirical; therefore to obtain a properresult the values appear in it should be expressed in the aforesaidterms.

[0019] It is preferable to use an X-ray tube with a rotating anode as asource of a soft X-rays.

[0020] The suggested invention is illustrated with figures:

[0021]FIG. 1 depicts a respective placing of the members of a device forlithography;

[0022]FIG. 2 depicts main constructive sizes of a half lens, used in thedevice.

[0023] An X-ray lithography device includes a source of a divergent softX-rays 1, which output aperture is placed in the focus of a half lens 2.A means (it is not shown in the figure) for a mask 3 placing is locatedon the side of an output (right one according to FIG. 1) face of a halflens 2. The said means is placed so that a plane of a mask 3 is parallelto an output face of a half lens 2, i.e. it should be perpendicular to alongitudinal axis of a half lens and to a axial line of an emergentquasi-parallel radiation beam, formed by a half lens.

[0024] A means (it is not shown in the figure) for a substrate 4 placingwith a layer of resist 5 applied on it is located after the means for amask placing. The means for a substrate placing should be located sothat a plane of a resist is parallel to a plane of a mask and it isspaced from the said mask at a minimum distance.

[0025] An absorbing filter 6 is placed between a source 1 and an inputface of a half lens 2. The absorbing filter 6 represents a product madeof material, absorbing an X-rays, as an axi-symmetric body, beingrelative to an axis combined with a longitudinal axis of a half lens. Athickness of the absorbing filter 6 is minimum in its peripheral partand it becomes thicker as approaching its central part, adjacent to anaxis of symmetry. A law of changing of thickness of an absorbing filter6 as a function of a distance from the axis of symmetry (the said axisof symmetry, being combined with a longitudinal axis of a lens 2 when afilter is placed) is chosen so that to obtain a uniform intensity of abeam along a cross section on the output of a lens. When the choice isexecuted a monitoring of radiation intensity distribution along thecross section of emergent beam of a specific half lens, intended forusage in a given lithography device, is realized by means of one orother detecting means while producing of the device. The said law ofchanging of thickness of an absorbing filter is of a character close toan exponential one. Such a filter can be made, in particular, as asubstrate, made of a light metal (for instance, aluminum) and appliedwith a layer of more heavy metal (for instance, copper or lead), withthickness, decreasing to a periphery.

[0026] The suggested X-ray lithography device works as follows.

[0027] A divergent radiation of a source 1 passes through an absorbingfilter 2, which attenuates it in dependence on the deflection angle fromthe longitudinal axis of a half lens 2 in inverse proportion toimpending attenuation when the radiation passes through the channels ofa half lens. A half lens 2 transforms a divergent input beam ofradiation to a quasi-parallel one. Owing to aforesaid character of theinput radiation attenuating the emergent radiation has an intensitydistribution along the cross section of a beam close to a uniform one(practical tolerable nonuniformity is 5-10%). This radiation afterpassing through the transparent sections of the mask 3 reaches theresist 2, applied on the surface of the substrate 4 to be irradiated. Asa result of an X-rays acting on the resist, sensitive to such radiation,“the windows” (free of resist sections of the substrate surface underirradiation) appear, the said windows form an image, repeating the imageof the mask 3.

[0028] From the point of view of imaging precision an actual divergenceof the emergent quasi-parallel radiation of the half lens 2 and thedistance between the mask and the resist are of essential importance, asthe image spreading is of the following order:

δ≈d·Δθ,  (2)

[0029] where

[0030] d is a distance between the mask 3 and the resist 5,

[0031] Δθ is a divergence angle of a quasi-parallel emergent radiationof a half lens 2.

[0032] In what follows the substrate 4 is etched wherein an etchant actson the surface under processing through the “windows” in the resist 5and it does not act on the other sections, a layer of resist stable tothe etchant retains on. As a whole this stage of a technological processdoes not differ from a traditional one (see, for instance, Encyclopedicdictionary “Electronics”, Moscow, “Soviet encyclopedia”, 1991 [5], pp.254-256).

[0033] As it was mentioned above in the invention description placingthe absorbing filter 6 on the path of radiations of source 1 to the lens2 protects a lens from an extra radiation. In experiments, carried outwith the usage of the power values of the source 1 discussed below, inthe absence of an absorbing filter before the input face of a half lensthe said face softening was observed. An absorbing filter is of no suchfine structure as an X-ray half lens, and it is much more thermalstrong. Besides a resist is a simple and cheap unit, which can beperiodically changed.

[0034] The suggested placing of an absorbing filter makes possible toexclude the action of a secondary scattered radiation, emerging in thesaid filter, on the resist. If an absorbing filter is placed on theoutput of a half lens 2 this radiation would cause much more spreading,than spreading defined according to the formula (2), as this radiationis not quasi-parallel and its divergence is not limited by a small angleΔθ and can reach 90°.

[0035] Let's make estimated calculations of possible coefficients andmain constructive parameters of the suggested X-ray lithography device.

[0036] As of now a diameter of plate-substrates about 30 cm (i.e. anarea is about 750 cm²) has been obtained. On the basis of desirableproductivity of processing of 10 such plates per an hour, the area to beprocessed will be 7500 cm². A sensitivity of present X-ray resists is of20 mJ/cm² order. It means that 7500·20=150 000 mJ=150 J of X-rays energyshould be delivered on the resist within 1 hour. A coefficient of powertransformation, consumed by an X-ray tube, to an X-rays is defined by aformula:

g=k(U−U _(k))^(1.5),  (3)

[0037] where

[0038] k≈10⁻⁴ (for X-rays with the quantum energy within the range understudy);

[0039] U_(k) is an ionization potential of the characteristic radiation[kV] under study;

[0040] U is voltage across the tube.

[0041] For instance, for K_(α) radiation of aluminum (E≈1.5 keV) atU−U_(k)=30 kV:

g=1.65·10⁻²,

[0042] i.e. a transformation coefficient is of 1% order. As of nowX-rays tubes with power consumption of 200 kW and more are produced. Atube not most high-power (30 kW) with a rotating anode will beconsidered to be used. Such tubes can work without repair within 10 000hours, thus the required repair is of minor nature as the operatingexperience implies. Their overall dimensions and weight are small. Atthe said tube power it is possible to obtain the following X-rays power

30·1.65·10⁻²=0.495 kW≈0.5 kW.

[0043] Thus it should be taken into account that at least half of thisenergy is absorbed in the rotating anode. An additional 30% falls on thehard part of the radiation, which should be filtered (such a filtrationis realized in the lens; see, for instance: M. A. Kumakhov. Channelingparticles radiation in crystals. Moscow, Energoatomizdat, 10986 [6], p.42). Altogether about 100 W from the said 0.5 kW will represent usefulpower, being emitted by an X-ray tube with a rotating anode. Howeverthis energy is emitted in a solid angle of 4π.

[0044] The walls of the channels for a radiation transporting of a halflens 2 are made of light metals or their oxides, light glasses, and soforth materials to provide good reflection and small absorption of usedsoft X-rays in the energy range 1-5 keV (if the inner surface of thechannels is coated the aforesaid refers to the material of coating). Anymaterials, including elements with an atomic weight not more than 22 andbeing acceptable for capillary lenses producing from the technologicalpoint of view, are appropriate. A half lens 2 is made of polycapillariesor as a monolithic lens according to the technology, described in thework: V. M. Andreevsky, M. V. Gubarev, P. I. Zhidkin, M. A. Kumakhov, A.V. Noskin, I. Yu. Ponomarev, Kh. Z. Ustok. X-ray waveguide system with avariable cross-section of the sections. The IV-th All-Union Conferenceon Interaction of Radiation with Solids. Book of Abstracts (May 15-19,1990, Elbrus settlement, Kabardino-Balkarian ASSR, USSR, pp. 177-178)[7].

[0045] If a radiation energy is E=1 keV according to the formula (1) wecan evaluate:

0.7≦ψ≦1.3.

[0046] Let's take ψ=60°≈1 radian. If ψ=60° a half lens captures about10% of isotropicly diverging radiation of a tube, i.e. approximately 10W. If a radiation rotation angle is equal to 30° relevant to arespective capture angle 60°, taking place in peripheral channels of alens, only about 5% of energy is transferred to a channel output. Excessof energy must be absorbed by a filter 6 before a radiation enters inthe channels, placed more close to a longitudinal axes of a half lens.So, it is necessary to recognize that only 5% of energy, captured by ahalf lens, reaches its output and forms approximately uniformquasi-parallel beam at a cross-section.

[0047] It means that 5·10⁻²·10 J=0.5 J=500 mJ falls on the mask per asecond. As it was described above it is necessary to obtain 150 J per anhour, i.e. 40 mJ per a second. So, the studied system, including a tubeof 30 kW power and a half lens with 60° capture angle, meets acceptedrequirements with safety from the point of view of energetics oflithographic process.

[0048] For main constructive sizes of a half lens (FIG. 2) one canderive the following formulas on the basis of relationships from thework: V. A. Arkadiev, M. A. Kumakhov. Concentration of synchrotronradiation with capillary focusing systems. Optic of beams, pp. 43-50.Institute for Roentgen Optical Systems. Moscow, 1993.

f=h/2tg(ψ/2),  (4)

h=H−2L[1−cos(ψ/2)]/sin(ψ/2),  (5)

R=L/sin(ψ/2),  (6)

[0049] where

[0050] ψ is a radiation capture angle;

[0051] f is a focal distance;

[0052] h is a an input diameter of a half lens;

[0053] H is an output diameter of a half lens;

[0054] L is a length of a half lens;

[0055] R is a curvature radius of a channel, being most distant from anoptical axis of a half lens.

[0056] According to the aforesaid suggestions (i.e. a diameter of aplate-substrate is 30 cm) an output diameter of a half lens shouldapproximate 30 cm as well. Let's accept, that a length of a half lens is30 cm as well. With regard to that one can calculate by the formulas(4)-(6) the values of the other sizes for a capture angle ψ=60°, definedabove:

[0057] an input diameter of a lens h=13.8 cm,

[0058] focal distance f=11.9 cm,

[0059] a curvature radius of a channel, being most distant from anoptical axis of a half lens, R=60 cm.

[0060] To estimate blurring of a mask image, transmitted on the resist,two factors, which define a divergence of an emergent radiation of ahalf lens 2, should be taken into account. The first factor is aradiation divergence on the input of each channel of a half lens of thefollowing order

Δθ₁ =l ₀ /f,  (7)

[0061] where

[0062] l₀ is an aperture size of an X-ray source,

[0063] f is a focal distance of a half lens 2.

[0064] The second factor is an out-of-parallelism of initially parallelrays after they were reflected from the curved wall of a channel. Aninfluence of this factor is maximum for the peripheral channels, beingthe most curved (minimum curvature radius), and for input rays, beingthe most distanced from each other (i.e. this distance is equal to adiameter d₀ of a channel). A divergence, caused by this factor, of anoutput radiation is equal to

Δθ₂=(2d ₀ /R)^(1/2),  (8)

[0065] where

[0066] d₀ is a diameter of a channel for a radiation transporting,

[0067] R is a curvature radius of a channel, being most distant from anoptical axis of a half lens.

[0068] To determine a resulting divergence, taking into account a randomand independent character of an influence of the said factors, let's addquadratically divergences, defined by the formulas (7) and (8):

Δθ=[(Δθ₁)²+(Δθ₂)²]^(1/2)=[(l ₀ /f)²+2d ₀ /R] ^(1/2).  9

[0069] So, if a focal distance is f=11.9 cm and a typical value 10=1 mmfor a diameter of the channels d₀=5 micron, obtained above, a divergenceis of Δθ≈10⁻² radian order.

[0070] At such a divergence and a typical distance value between a mask3 and a layer of a resist d=20 micron an image blurring, defined by theformula (2), is of δ=0.2 micron order.

[0071] The obtained results testify that a submicron resolution usingplate-substrates of large (up to 30 cm) sizes without stepping andprocessing efficiency up to 10 plates per an hour can be realized bymeans of the suggested device.

[0072] The usage of radiation energy more than 1 keV makes possible touse resists up to 1 mm thick, realizing spatial structures on the basisof LIGA-technology.

SOURCES

[0073] 1. Book of Abstracts. 3^(rd) All-Union Conference on RelativisticParticles Radiation in Crystals. May 25-30, 1988. (Nalchik, 1988).

[0074] 2. Multiple reflection from surface X-ray optics (M. A. Kumakhovand F. F. Komarov. PHYSICS REPORTS. A Review Section of Physics Letters,volume 191, number 5, August 1990. North-Holland.

[0075] 3. U.S. Pat. No. 5,175,755 (published 29.12.92).

[0076] 4. M. A. Kumakhov. State and perspectives of capillary Roentgenoptics. Proceedings of SPIE—The International Society for OpticalEngineering. Volume 2011, Jul. 14-16, 1993, San Diego, Calif.

[0077] 5. Encyclopedic dictionary “Electronics”, Moscow, “Sovietencyclopedia”, 1991, pp. 254-256.

[0078] 6. M. A. Kumakhov. Channeling particles radiation in crystals.Moscow, Energoatomizdat, 1986.

[0079] 7. V. M. Andreevsky, M. V. Guvarev, P. I. Zhidkin, M. A.Kumakhov, A. V. Noskin, I. Yu. Ponomarev, Kh. Z. Ustok. X-ray waveguidesystem with a variable cross-section of the sections. The IV-thAll-Union Conference on Interaction of Radiation with Solids. Book ofAbstracts (May 15-19, 1990, Elbrus settlement, Kabardino-Balkarian ASSR,USSR, pp. 177-178).

[0080] 8. V. A. Arkadiev, M. A. Kumakhov. Concentration of synchrotronradiation with capillary focusing systems. Optic of beams, pp. 43-50.Institute for Roentgen Optical Systems. Moscow, 1993.

What is claimed is:
 1. A contact lithography device, comprising a softX-rays source, a half lens for transforming a divergent radiation ofthis source to a quasi-parallel one including a set of channels forradiation transporting with a total external reflection oriented along ageneratrix of barrel-shaped surfaces, the means for a mask and asubstrate with a resist applied on it placing spaced from the side of anoutput face of a half lens, and an absorbing filter for smoothing anirregularity of a beam intensity of an emergent radiation of a half lenspresenting an intensity attenuation from the center to the periphery ofa beam, wherein an absorbing filter is placed between a radiation sourceand an input face of a half lens, a relationship of cross-sections of ahalf lens and its focal distance from the side of the input is chosenaccording to provide a condition of capturing of a source radiation inthe following limits: 0.7/E ^(1.5)≦ψ≦1.3/E ^(1.5),  where ψ is an angleof a radiation capture [rad]; E is radiation energy of the used source[keV], thus a material of a reflecting surface of the channels forradiation transporting includes elements with an atomic number not morethan 22, and radiation energy of the used source is 0.6-6 keV.
 2. Adevice according to claim 1 wherein it includes an X-ray tube with arotating anode as source of soft X-rays.